In oxide superconductors, since the critical temperature (Tc) thereof exceeds the temperature of liquid nitrogen, their applications to wires, devices, etc. are expected, and various studies have been vigorously made.
In particular, in order to apply oxide superconductors to wires, it is necessary to produce oxide superconductor shaving a high critical current density (Jc) and a long length. On the other hand, in order to obtain long oxide superconductors in the form of a tape, it is necessary to form an oxide superconductor on a metal tape from the viewpoints of strength and flexibility.
Also, since oxide superconductors have anisotropy in the crystallography, in order to improve the Jc, it is necessary to establish a film formation process for epitaxially growing an oxide superconductor on an aligned substrate.
As processes for producing a tape-formed RE based oxide superconductor, i.e., RE1+xBa2−xCu3Oy (wherein RE represents at least one element selected from the group consisting of Y, Nd, Sm, Gd, Eu, Yb, Pr and Ho; x represents the number of 0≦x≦0.4; and y represents the number of 6.5≦y≦7.0, hereinafter the same) based oxide superconductor, there is known the MOD process.
This MOD process (Metal Organic Deposition process) is to thermally decompose organic acid metal salts and is a process for coating a solution having a metal component-containing organic compound uniformly dissolved therein on a substrate and then heating it for thermal decomposition, thereby forming a thick film on the substrate. The MOD process has such an advantage that since not only a high Jc is obtained in a non-vacuum process, but also high-speed film formation can be achieved at low costs, this process is suitable for the production of oxide superconducting wires in the form of a tape.
In the MOD process, when a starting material containing metal organic acid salts is thermally decomposed, a carbonate of an alkaline earth metal (such as Ba) is usually formed. In the formation of an oxide superconductor by a solid phase reaction via such a carbonate, a high-temperature heat treatment at 800° C. or higher is necessary. Also, a lowering of the Jc caused due to film thickening becomes a serious problem.
Against the foregoing problems, in recent years, there have been vigorously carried out processes for forming an RE (123) superconductor (RE:Ba:Cu=1:2:3, hereinafter the same) using fluorine-containing organic acid salts (for example, a TFA salt: trifluoroacetic acid salt) as the starting material by a heat treatment in a water vapor atmosphere while controlling a water vapor partial pressure (for example, see Patent Document 1). According to the process using this TFA salt as the starting material, it is possible to epitaxially grow the RE (123) superconductor from a substrate by reaction of water vapor and a fluorine-containing amorphous precursor. Concretely, after coating the starting material solution on the substrate, the coated substrate is thermally treated for crystallization at 750° C. via a calcination step for thermally treating it at not higher than 400° C. for the purpose of decomposing organic components, thereby forming a superconducting film. As characteristics of the heat treatment in the present process, there is enumerated the generation of an HF gas by reaction of fluorine and water vapor in the film. In particular, in the calcination step, a large amount of HF gas is generated due to the decomposition of copper trifluoroacetate into CuO. For this reason, in order to prevent cracking, etc. in the film due to abrupt decomposition reaction from occurring, it is necessary to control the heating rate in the calcination step at not more than 1° C./min. Also, taking into consideration the solubility of the TFA salt, highly volatile methanol must be chosen as a solvent. Accordingly, a change of the starting material solution with time is vigorous even at room temperature, and its viscosity becomes higher with an increase of the concentration.
In the light of the above, since the trifluoroacetic acid metal salt is soluble in only a low boiling solvent such as methanol, the solvent is vaporized during the coating work of the starting material solution on the substrate so that uniform coating cannot be achieved. Thus, the uniformity of the resulting thick oxide superconducting film was low. Also, since it takes a long period of time for the calcination step prior to the actual baking, the high-speed film formation was difficult.
Accordingly, an object of the present invention is to provide a composition for a thick oxide superconducting film capable of being subjected to film formation with uniformity at a high speed, which is suitable for producing thick copper based oxide superconducting films by the MOD process, and an oxide superconductor in the form of a thick film tape which is subjected to film formation with uniformity at a high speed using the subject composition for a thick oxide superconducting film.
[Patent Document 1] U.S. Pat. No. 5,231,074